Internal combustion engine exhaust emissions, and especially diesel engine exhaust emissions, have recently come under scrutiny with the advent of stricter regulations, both in the U.S. and abroad. While diesel engines are known to be more economical to run than spark-ignited engines, diesel engines inherently suffer disadvantages in the area of emissions. For example, in a diesel engine, fuel is injected during the compression stroke, as opposed to during the intake stroke in a spark-ignited engine. As a result, a diesel engine has less time to thoroughly mix the air and fuel before ignition occurs. The consequence is that diesel engine exhaust contains incompletely burned fuel known as particulate matter, or “soot”. In addition to particulate matter, internal combustion engines including diesel engines produce a number of combustion products including hydrocarbons (“HC”), carbon monoxide (“CO”), nitrogen oxides (“NOx”), and sulfur oxides (“SOx”). Aftertreatment systems may be utilized to reduce or eliminate emissions of these and other combustion products.
FIG. 1A shows a block diagram providing a brief overview of a vehicle powertrain. The components include an internal combustion engine 20 in flow communication with one or more selected components of an exhaust aftertreatment system 24. The exhaust aftertreatment system 24 optionally includes a catalyst system 96 upstream of a particulate filter 100. In the embodiment shown, the catalyst system 96 is a diesel oxidation catalyst (DOC) 96 coupled in flow communication to receive and treat exhaust from the engine 20. The DOC 96 is preferably a flow-through device that includes either a honeycomb-like or plate-like substrate. The substrate has a surface area that includes (e.g., is coated with) a catalyst. The catalyst can be an oxidation catalyst, which can include a precious metal catalyst, such as platinum or palladium, for rapid conversion of hydrocarbons, carbon monoxide, and nitric oxides in the engine exhaust gas into carbon dioxide, nitrogen, water, or NO2.
Once the exhaust has flowed through DOC 96, the diesel particulate filter (DPF) 100 is utilized to capture unwanted diesel particulate matter from the flow of exhaust gas exiting engine 20, by flowing exhaust across the walls of DPF channels. The diesel particulate matter includes sub-micron sized solid and liquid particles found in diesel exhaust. The DPF 100 can be manufactured from a variety of materials including but not limited to cordierite, silicon carbide, and/or other high temperature oxide ceramics.
The treated exhaust gases can then proceed through a compartment containing a diesel exhaust fluid (DEF) doser 102 for the introduction of a reductant, such as ammonia or a urea solution. The exhaust gases then flow to a selective catalytic reduction (SCR) system 104, which can include a catalytic core having a selective catalytic reduction catalyst (SCR catalyst) loaded thereon.
System 24 can include one or more sensors (not illustrated) associated with components of the system 24, such as one or more temperature sensors, NOx sensor, NH3 sensor, oxygen sensor, mass flow sensor, particulate sensor, and a pressure sensor.
As discussed above, the exhaust aftertreatment system 24 includes a Selective Catalytic Reduction (SCR) system 104. The SCR system 104 includes a selective catalytic reduction catalyst which interacts with NOx gases to convert the NOx gases into N2 and water, in the presence of an ammonia reductant. The overall reactions of NOx reductions in SCR are shown below.4NO+4NH3+O2→4N2+6H2O  (1)6NO2+8NH3→7N2+12H2O  (2)2NH3+NO+NO2→2N2+3H2O  (3)
Where Equation (1) represents a standard SCR reaction and Equation (3) represents a fast SCR reaction.
The performance of the SCR catalyst is often counterbalanced by catalyst durability. This challenge is further compounded by the increasingly stringent emissions regulatory demands on the one hand, and the economic pressure surrounding fuel economy on the other. Furthermore, the performance of the SCR catalyst is influenced by the level of engine out NOx (EO NOx) that has to be processed by the SCR catalyst. The current trend is in the direction of higher engine out NOx to improve fuel economy, while emission levels are simultaneously being reduced. For example, at present, EO NOx can reach as high as 7 g/kW-hr for at least a short period of time. However, it is anticipated that in the future, there will be a move towards very low tailpipe NOx (e.g., decreasing from about 0.2 to about 0.02 g/kW-hr).
High EO NOx has been shown to result in urea deposit build up in the SCR, due to the extremely high levels of diesel exhaust fluid that is introduced into the system, and insufficient residence time for complete decomposition to form NH3. The formation and accumulation of urea deposits on the SCR catalyst can result in severe damage to both the chemical and physical integrity of the SCR coating. Furthermore, the high intensity of diesel exhaust fluid dosing and the relatively long duration of the dosing in urea decomposition reactor 102 can result in large quantities of water being released onto the SCR catalyst. As the SCR catalyst can be primarily composed of zeolites, which are powerful water adsorbing materials, the quantities of water can present a problem with both durability and cold start performance of the SCR catalyst.
At low EO NOx conditions, challenges are similar to those present under extended idling and cold start conditions. In other words, when SCR temperatures are too low for diesel exhaust fluid dosing and normal SCR operation (between about 250-450° C.), other strategies are required to meet emissions standards.
Without wishing to be bound by theory, it is believed that the advent of engine gas recirculation (EGR) has resulted in reduced peak in-cylinder temperatures for combustion to reduce engine out NOx. The reduced peak in-cylinder temperatures are highly desirable from an emissions control perspective. However, the lower peak in-cylinder temperatures also result in undesirable lower fuel economy. The reduced engine exhaust temperatures that result from increasing use of EGR also have a negative impact on cold-start conditions for engine aftertreatment system (EAS) performance. Effective emissions control by EAS requires temperatures of at least 200° C. to be attained before DEF dosing may commence. Therefore, during the EAS heat-up period under cold-start conditions (i.e., at temperatures of less than 200° C.), there is no emissions control.
Some challenges that are encountered in emissions control include:
(1) Cold-start conditions with relatively low engine exhaust temperatures can be addressed by close coupling the SCR to the engine to achieve maximum heat-up rate, with exposure of the SCR catalyst to non-pretreated exhaust directly from the engine. However, only partial NOx reduction can be achieved in this manner. Therefore, a second downstream SCR (or a SCRF) will be required.
(2) Increased system size and complexity arise when the EAS includes a close coupled zeolite-based SCR, therefore, a DOC upstream of the SCR is required for NO2-make for optimal performance, with a DEF doser and an ammonia slip catalyst (ASC), also called an ammonia oxidation catalyst (AMOX), downstream of the SCR to decrease NH3 slip into the DOC. In some embodiments, while a close coupled vanadia-based SCR would not require a DOC upstream of the SCR, there exists a risk of sublimed vanadium escaping into the environment.
(3) Space limitations for close coupling requires that the EAS be made more compact, for example, by combining SCR and DPF to form a SCRF, which presents the following challenges:                (i) Competition between the fast SCR reaction and soot oxidation reaction for the available NO2 from the DOC;        (ii) No passive soot oxidation, because platinum group metals (PGMs) cannot be used on the DPF substrate due to the presence of NH3 for the SCR reaction. Oxidation of NH3 with PGMs also produces N2O, which is an undesirable greenhouse gas.        (iii) Finally, the reduced ash loading capacity of the SCRF relative to a DPF and the associated higher pressure change (AP) dictate a shorter ash cleaning interval and cost of ownership for the customer.        
(4) The potential for increased poisoning and hydrothermal aging of EAS catalysts are a major concern that arises from both close coupling of the SCR and in particular, when SCR on DPF (i.e., SCRF) technologies are employed.
(5) Increasingly stringent emissions regulations are expected to be enforced by the year 2021; including tailpipe (TP) NOx≤0.02 g/kw-hr, lower N2O emissions standards, and generally tightened greenhouse gas regulation.
Thus, there is a need for engine aftertreatment catalysts that can address the challenges facing emission control. The present disclosure seeks to fulfill these needs and provides further related advantages.